Brake control arrangement and method for braking a motor vehicle

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method for braking a motor vehicle. The invention also relates to a brake control arrangement for a motor vehicle according to the preamble of claim 7. Furthermore, the invention relates to a computer program product comprising computer program code for implementing a method according to the invention, and an electronic control unit.

BACKGROUND ART

Heavy motor vehicles, such as lorries, towing vehicles and buses, are often provided with a brake control arrangement operable in an automatic braking mode, which can be activated by the driver of the vehicle when the driver wants to maintain an essentially constant speed of the vehicle in a downhill slope de- scended by the vehicle. A reference speed for the automatic braking operation is settable by the driver. In dependence on information as to the set reference speed and the actual speed of the vehicle, the brake control arrangement will automatically control one or several auxiliary brakes of the vehicle so as to en- deavour to restrict the speed of the vehicle to the reference speed when the automatic braking operation is initiated. An auxiliary brake is a supplement to the ordinary frictional wheel brakes of the vehicle and is configured to produce a retarding torque for decreasing the rotational speed of the driven wheels of the vehicle by reducing, directly or indirectly, the rotational speed of the propeller shaft of the vehicle. An auxiliary brake may act directly on the propeller shaft or on the vehicle engine or the engine output shaft. An auxiliary brake may for instance be a compression brake, an exhaust brake, an electromagnetic retarder or a hydrodynamic retarder. The auxiliary brakes are with advantage used for constant speed keeping purposes in downhill slopes and for moderate decelerations, whereas the vehicle's or-

dinary wheel brakes are mainly used for powerful and sudden braking operations. With the use of auxiliary brakes, the ordinary wheel brakes, which normally constitute frictional brakes of disc brake or drum brake type, are prevented from overheating with the associated risk of brake failure and the wear thereof is reduced.

An automatic braking operation for keeping a constant speed of a heavy motor vehicle usually involves the control of the braking effect of a hydrodynamic retarder, which for instance acts on the propeller shaft of the vehicle. Such a retarder is provided with a braking medium, for instance in the form of oil, which is normally cooled by means of a retarder cooler connected to the ordinary cooling system of the vehicle. A large amount of heat is gener- ated in the retarder when it is in operation, i.e. when it is exerting a breaking effect. This heat is to be removed via the retarder cooler and the ordinary cooling system of the vehicle. To avoid overheating of the cooling system, the breaking effect of the retarder is reduced by the control unit which is controlling the op- eration of the retarder when the heating level of the braking medium is so high that the cooling system is unable to remove a sufficient amount of the generated heat. A method for controlling the braking effect of a hydrodynamic retarder in dependence on the available cooling effect in the associated cooling system is for instance known from DE 197 16 919 A1 .

An auxiliary brake only acts, directly or indirectly, on the propeller shaft of the vehicle. Thus, there is a risk of skidding sideways with the vehicle when braking on a slippery road with auxiliary brakes, and when the vehicle comprises a trailer there is a risk of the occurrence of a dangerous so-called jack knife effect. There is therefore a need to reduce the braking effect of the auxiliary brakes when any of the wheels braked by means thereof is subjected to skidding relative to the road surface. WO 02/32737 A1 discloses an arrangement and a method for controlling an auxiliary brake of a vehicle, where the braking effect called for from the auxiliary brake is variably restricted in dependence on a pa-

rameter related to the degree of skidding of any of the braked wheels relative to the road surface.

DISCLOSURE OF THE INVENTION

The object of the present invention is to propose a new and advantageous manner of controlling an automatic braking operation of a motor vehicle.

According to the invention, this object is achieved by a method having the features defined in claim 1 and a brake control arrangement having the features defined in claim 7.

According to the invention, one or several auxiliary brakes of the vehicle are automatically controlled during an initiated automatic braking operation so as to endeavour to restrict the speed of the vehicle to a reference speed, and frictional wheel brakes of the vehicle are automatically activated to apply a braking effect on at least one pair of wheels of the vehicle when the following conditions are simultaneously fulfilled during an ongoing automatic braking operation:

• it is established that the speed of the vehicle exceeds a given threshold speed, which threshold speed is higher than the refer- ence speed, and

• it is established that the braking power of one of said one or several auxiliary brakes is restricted in such a manner that this auxiliary brake is not allowed to exert the maximum braking power exertable by it at the prevailing rotational speed of the en- gine of the vehicle or the prevailing rotational speed of the propeller shaft of the vehicle.

Thus, the speed of the vehicle is allowed to increase to the threshold speed when the braking power of an auxiliary brake in- volved in the automatic braking operation is restricted, for instance due to an imminent overheating condition in a cooling system associated with a hydrodynamic retarder or a detected

skidding of any of the braked wheels. In order to prevent the vehicle from accelerating to reach an unpleasantly or dangerously high speed, frictional wheel brakes of the vehicle are automatically activated when the speed of the vehicle has reached the threshold speed so as to thereby assist the auxiliary brake or brakes in retarding the vehicle. If the auxiliary brake or brakes is/are capable of restricting the speed of the vehicle so as to prevent the vehicle from accelerating to reach the threshold speed despite the fact that the braking power of an auxiliary brake has been restricted, the frictional wheel brakes will not be activated. Hereby, an unnecessary activation of the frictional wheel brakes is avoided. Furthermore, the acceleration from the reference speed to the threshold speed will allow the driver of the vehicle to perceive that the auxiliary brake or brakes is/are not capable of restricting the speed of the vehicle to the reference speed and the driver may then take appropriate actions in consequence thereof.

According to an embodiment of the invention, the threshold speed is set to a value corresponding to the reference speed plus a given offset value, for instance an offset value in the order of 5-10 km/h. Hereby, the acceleration of the vehicle from the reference speed to the threshold speed will be clearly perceivable by the driver at the same time as the acceleration is kept within acceptable safety margins.

According to another embodiment of the invention, the magnitude of said offset value is established in dependence on the magnitude of an evaluation value representing the degree of curvature of the latest stretch of road travelled by the vehicle. A larger offset value, and consequently a higher speed increase of the vehicle, may be allowed when the vehicle is travelling on a straight road as compared to the case when the vehicle is travelling on a curvy road. This is due to the fact that an unexpectedly high ve- hide speed may be unpleasant and even dangerous when the vehicle is to turn in a sharp road curve.

According to another embodiment of the invention, said frictional wheel brakes, after having been automatically activated, are automatically controlled so as to make the vehicle decelerate back to the reference speed. Hereby, it is secured that the speed increase above the reference speed only is temporary.

According to another embodiment of the invention, said frictional wheel brakes, after having been automatically activated, are deactivated when it is established that the vehicle has been decel- erated back to the reference speed. Hereby, a longer unexpected activation of the frictional wheel brakes with the associated wear thereof is avoided and the auxiliary brake or brakes involved in the automatic braking operation is/are given a new chance to keep the vehicle speed restricted to the reference speed.

Further advantageous features of the method and the brake control arrangement according to the invention are indicated in the dependent claims and the following description.

The invention also relates to a computer program product having the features defined in claim 13 and an electronic control unit having the features defined in claim 15.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, a specific description of preferred embodiments of the invention cited as examples follows below. In the drawings:

Fig 1 is a schematic diagram of selected parts of a motor vehicle, illustrating an embodiment of a brake control arrangement according to the present invention,

Fig 2 is a schematic outline diagram of an electronic control unit for implementing a method according to the invention, and

Fig 3 is a flow diagram illustrating a method according to an embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION

Fig 1 schematically illustrates selected parts of a heavy motor vehicle 1 with a brake control arrangement 20 according to the present invention. The vehicle 1 comprises pneumatically operable frictional wheel brakes 2. The driving wheels 3 as well as the non-driven wheels 4a, 4b of the vehicle are each provided with its own frictional wheel brake 2. The frictional wheel brakes 2 may be of conventional disc brake or drum brake type and each of them may for instance comprise a conventional brake cylinder (not shown) operable to apply a braking effect on the associated wheel 3, 4a, 4b so as to thereby retard the vehicle.

The vehicle 1 comprises a vehicle engine 5, for instance in the form of a conventional combustion engine, such as a diesel engine. The engine 5 is connected to the driving wheels 3 of the vehicle via a driveline, which comprises inter alia a gearbox 6 and a propeller shaft 7. The driveline is arranged to transfer the torque exerted by the engine 5 to the driving wheels 3.

The vehicle 1 is provided with an auxiliary brake in the form of a hydrodynamic retarder 8, which is here arranged to act on the propeller shaft 7. When activated, this retarder 8 exerts a braking power on the propeller shaft 7 and consequently on the driven wheels 3. Another auxiliary brake in the form of an exhaust brake 9 is arranged in connection with the engine 5. The vehicle may also be provided with other types of auxiliary brakes, such as for instance a compression brake and/or an electromagnetic retarder, in addition to the hydrodynamic retarder 8 and the exhaust brake 9 or as a substitute for any of them.

A cooling system for cooling the engine 5 and the retarder 8 is schematically indicated at 10.

The brake control arrangement 20 comprises brake control means 21 adapted to control the auxiliary brakes 8, 9 of the vehicle in dependence on information as to a reference speed s _ref and the actual speed of the vehicle so as to endeavour to restrict the speed of the vehicle to the reference speed s _ref when the vehicle descends a downhill slope with an automatic braking operation activated. Thus, this automatic braking operation constitutes a constant speed keeping function.

The automatic braking operation may for instance be activated and initiated by the driver of the vehicle 1 by means of a manoeuvring member 22, for instance in the form of a lever or a de- pressible switch, which is to be actuated by the driver when the vehicle is running in a downhill slope at a speed which the driver wants to maintain. By actuation of the manoeuvring member 22, the prevailing speed of the vehicle is entered as the value of the reference speed s _ref and the automatic braking operation is initiated. The automatic braking operation may alternatively be activated and initiated when the driver depresses or releases the brake pedal of the vehicle.

The automatic braking operation may also be associated with a so-called cruise control system, also denominated speed control system, which comprises engine control means adapted to auto- matically control the engine 5 in dependence on information as to a target speed and the actual speed of the vehicle so as to endeavour to keep the speed of the vehicle essentially equal to the target speed and which also comprises means for initiating an automatic braking operation when the vehicle has been acceler- ated under the effect of gravity in a downhill slope to a speed above the target speed. The cruise control system comprises speed setting means, by means of which the driver of the vehicle may set the target speed to a desired value. The speed setting means may be configured to allow the driver to set the reference speed s _rθf for the automatic braking operation as a desired offset from the target speed. The speed setting means may also be configured to allow the driver to directly set a desired value for

the reference speed s _ref . It is realized that the reference speed s _ref always should be set higher than the target speed in order to avoid conflicts between the engine control means and the brake control means.

The actual speed of the vehicle 1 , i.e. the prevailing speed at which the vehicle is running, may be measured or estimated in any suitable manner. In the example illustrated in Fig 1 , the actual vehicle speed is established by means of a measuring mem- ber 1 1 arranged to generate a value representing the rotational speed of the propeller shaft 7. The brake control means 21 is arranged to receive information from this measuring member 11 .

The brake control means 21 is adapted to automatically activate frictional wheel brakes 2 of the vehicle to apply a braking effect on at least one pair of wheels 3, 4a, 4b of the vehicle when the brake control means 21 has established or received information indicating that the following conditions are simultaneously fulfilled during an ongoing automatic braking operation: • the speed of the vehicle exceeds a given threshold speed T _s , which threshold speed is higher than the reference speed s _ref , and

• the braking power of one of said auxiliary brakes 8, 9 is restricted in such a manner that this auxiliary brake is not allowed to exert the maximum braking power exertable by it at the prevailing rotational speed of the engine 5 or the prevailing rotational speed of the propeller shaft 7, i.e. the braking power of one the auxiliary brakes 8, 9 is limited to a value below the maximum exertable braking power of the auxiliary brake in ques- tion.

The brake control means 21 may be arranged to activate said frictional wheel brakes 2 directly or, as illustrated in Fig 1 , via a separate brake control unit 23 which controls the operation of the frictional wheel brakes.

The maximum braking power exertable by the retarder 8 depends on the rotational speed of the propeller shaft 7 and values representing this maximum braking power for different rotational speeds of the propeller shaft are established in advance and for instance stored as a look-up table or a calculation model on a data storage medium in the brake control means 21. There are different situations when the retarder 8 is not allowed to exert its maximum braking power. The braking power of the retarder 8 may for instance be restricted to a value below the maximum braking power in a known manner when it is established that the heating level of the reader's braking medium is so high that the cooling system 10 is unable to remove a sufficient amount of the generated heat. A possible implementation of such a restriction to the braking power of a retarder in connection with the detec- tion of an imminent overheating situation is more closely described in DE 197 16 919 A1. The braking power of the retarder 8 may also be restricted to a value below the maximum braking power in a known manner when it is established that any of the driven wheels 3 braked by means of the retarder is subjected to or is about to be subjected to skidding relative to the road surface. A possible implementation of such a restriction to the braking power of a retarder in connection with an imminent or detected wheel skidding is more closely described in WO 02/32737 A1 , which is hereby incorporated by reference.

The maximum braking power exertable by the exhaust brake 9 depends on the rotational speed of the vehicle engine 5 and values representing this maximum braking power for different rotational speeds of the engine are established in advance and for instance stored as a look-up table or a calculation model on a data storage medium in the brake control means 21 . The braking power of the exhaust brake 9 may for instance be restricted to a value below the maximum braking power in a known manner when it is established that any of the driven wheels 3 braked by means of the exhaust brake is subjected to or is about to be subjected to skidding relative to the road surface. A possible implementation of such a restriction to the braking power of an ex-

haust brake 9 in connection with an imminent or detected wheel skidding is more closely described in WO 02/32737 A1.

The brake control arrangement 20 is with advantage provided with processing means 24 adapted to set the threshold speed T _s to a value corresponding to the reference speed s _ref plus a given offset value, for instance an offset value in the order of 5-10 km/h. The offset value may be a fixed value. However, the processing means 24 may alternatively be adapted to establish the magnitude of said offset value in dependence on the magnitude of the reference speed s _rβf and/or in dependence on the magnitude of an evaluation value V _ev representing the degree of curvature of the latest stretch of road travelled by the vehicle.

The latest stretch of road that is taken into account in the establishment of the above-mentioned evaluation value V _ev may for instance be the stretch of road travelled by the vehicle during a time period of a given length up to the prevailing moment. The latest stretch of road that is taken into account in the establish- ment of the evaluation value V _ev may alternatively be defined as a distance of a given length travelled by the vehicle. The evaluation value V _ev may for instance be established based on measuring values from a sensor arranged to measure the rotation of the vehicle's steering wheel or based on the mutual difference between the measured or calculated rotational speeds of the vehicle's two front wheels 4a.

After having automatically activated said frictional wheel brakes 2, the brake control means 21 is suitably adapted to control these frictional wheel brakes so as to make the vehicle 1 decelerate back to the reference speed s _ref and to deactivate these frictional wheel brakes when it has established or received information indicating that the vehicle has been decelerated back to the reference speed.

An ongoing automatic braking operation may be terminated in a conventional manner, for instance when it is established that the

driver has depressed the accelerator pedal or the brake pedal of the vehicle.

The brake control means 21 and the processing means 24 may be integrated in one and the same electronic control unit 25, as illustrated in Fig 1 , but may alternatively be arranged in separate and mutually communicating electronic control units.

A flow diagram illustrating an embodiment of an inventive method for braking a motor vehicle is shown in Fig 3. In a first step S1 , an automatic braking operation is initiated. One or several auxiliary brakes 8, 9 of the vehicle are automatically controlled during the initiated automatic braking operation so as to endeavour to restrict the speed of the vehicle to a reference speed s _rβf . In a second step S2, it is established whether or not the speed of the vehicle exceeds a given threshold speed T _s , which threshold speed is higher than the reference speed s _ref . If this is not the case, step S2 is repeated after a certain time interval. If the vehicle speed exceeds the threshold speed, it is established in step S3 whether or not the braking power of any one of said one or several auxiliary brakes 8, 9 is restricted in such a manner that this auxiliary brake is not allowed to exert the maximum braking power exertable by it at the prevailing rotational speed of the engine of the vehicle or the prevailing rotational speed of the propeller shaft of the vehicle. If this is not the case, the process returns to step S2. If it is established in step S3 that the braking power of an auxiliary brake is restricted, frictional wheel brakes 2 of the vehicle are automatically activated in step S4 to apply a braking effect on at least one pair of wheels 3, 4a, 4b of the ve- hide so as to thereby decelerate the vehicle back to the reference speed s _rθf . Steps S2-S4 may then be repeated as long as the automatic braking operation is active. Steps S2 and S3 may of course also be carried out in the opposite order or essentially simultaneously.

Computer program code for implementing a method according to the invention is suitably included in a computer program, which is

loadable into the internal memory of a computer, such as the internal memory of an electronic control unit of a motor vehicle. Such a computer program is suitably provided via a computer program product comprising a data storage medium readable by an electronic control unit, which data storage medium has the computer program stored thereon. Said data storage medium is for instance an optical data storage medium in the form of a CD- ROM disc, a DVD disc etc, a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc, or a memory of the type ROM, PROM, EPROM or EEPROM or a Flash memory.

A computer program product according to an embodiment of the invention comprises computer program code for causing an elec- tronic control unit in a motor vehicle provided with frictional wheel brakes and at least one auxiliary brake:

- to determine or receive a value for a reference speed s _ref ;

- to determine or receive a value representing the actual speed of the vehicle; - to control one or several auxiliary brakes of the vehicle in dependence on the reference speed and the actual speed of the vehicle so as to endeavour to restrict the speed of the vehicle to the reference speed s _ref when an automatic braking operation has been activated; and - to activate frictional wheel brakes of the vehicle to apply a braking effect on at least one pair of wheels of the vehicle when it is established that the following conditions are simultaneously fulfilled during an ongoing automatic braking operation:

• the speed of the vehicle exceeds a given threshold speed T _s , which threshold speed is higher than the reference speed s _ref , and

• the braking power of one of said one or several auxiliary brakes is restricted in such a manner that this auxiliary brake is not allowed to exert the maximum braking power exertable by it at the prevailing rotational speed of the engine of the vehicle or the prevailing rotational speed of the propeller shaft of the vehicle.

Fig 2 very schematically illustrates an electronic control unit 25 comprising an execution means 31 , such as a central processing unit (CPU), for executing computer software. The execution means 31 communicates with a memory 33, for instance of the type RAM, via a data bus 32. The control unit 25 also comprises data storage medium 34, for instance in the form of a memory of the type ROM, PROM, EPROM or EEPROM or a Flash memory. The execution means 31 communicates with the data storage medium 34 via the data bus 32. A computer program comprising computer program code for implementing a method according to the invention is stored on the data storage medium 34.

The invention is of course not in any way restricted to the em- bodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.